ALC Parameter Definitions. Nuvoton NAU88C22

PGA Input

PGA Output

PGA Gain

Figure 7: ALC Normal Mode Operation

3.9.2 ALC Parameter Definitions

Automatic level and volume control features are complex and have difficult to understand traditional names for many features and controls. This section defines some terms so that the explanations of this subsystem are more clear.

ALC Maximum Gain: Register 32 (ALCMXGAIN) This sets the maximum allowed gain in the PGA during normal mode ALC operation. In the Limiter mode of ALC operation, the ALCMXGAIN value is not used. In the Limiter mode, the maximum gain allowed for the PGA is set equal to the pre-existing PGA gain value that was in effect at the moment in time that the Limiter mode is enabled.

ALC Minimum Gain: Register 32 (ALCMNGAIN) This sets the minimum allowed gain in the PGA during all modes of ALC operation. This is useful to keep the AGC operating range close to the desired range for a given application scenario.

ALC Target Value: Register 33 (ALCSL) Determines the value used by the ALC logic decisions comparing this fixed value with the output of the ADC. This value is expressed as a fraction of Full Scale (FS) output from the ADC. Depending on the logic conditions, the output value used in the comparison may be either the instantaneous value of the ADC, or otherwise a time weighted average of the ADC peak output level.

ALC Attack Time: Register 34 (ALCATK) Attack time refers to how quickly a system responds to an increasing volume level that is greater than some defined threshold. Typically, attack time is much faster than decay time. In the NAU88C22, when the absolute value of the ADC output exceeds the ALC Target Value, the

PGA gain will be reduced at a step size and rate determined by this parameter. When the peak ADC output is at least 1.5dB lower than the ALC Target Value, the stepped gain reduction will halt.

ALC Decay Time: Register 34 (ALCDCY) Decay time refers to how quickly a system responds to a decreasing volume level. Typically, decay time is much slower than attack time. When the ADC output level is below the

ALC Target value by at least 1.5dB, the PGA gain will increase at a rate determined by this parameter. The decay time constant is determined by the setting in register 34, bits 4 to 7 (ALCDCY), which sets the delay between increases in gain. In Limiter mode, the time constants are faster than in ALC mode. (See Detailed

Register Map.)

ALC Hold Time Register 33 (ALCHLD) Hold time refers to a duration of time when no action is taken. This is typically to avoid undesirable sounds that can happen when an ALC responds too quickly to a changing input signal. The use and amount of hold time is very application specific. In the NAU88C22, the hold time value is the duration of time that the ADC output peak value must be less than the target value before there is an actual gain increase.

NAU88C22 Datasheet Rev 0.6 Page 24 of 101 June, 2016